It is generally known to operate motor vehicles with an internal combustion engine and a transmission in so-termed start-stop operation, in which the internal combustion engine, perhaps during a prolonged stop at a traffic light, is turned off and started up again, for example, by actuating the accelerator pedal.
If an automatic transmission is used in such a vehicle, the problem arises that when the internal combustion engine is turned off, the pressure in the hydraulic system of the transmission also falls to too low a level, since a hydraulic pump mechanically driven by the combustion engine can no longer maintain hydraulic pressure when the combustion engine has stopped.
As is known, however, in automatic transmissions a sufficiently high hydraulic pressure is needed, for example to actuate transmission shift elements such as clutches and/or brakes, with whose help transmission ratio changes in the transmission are carried out. Just as important, is to maintain a hydraulic pressure high enough to keep the switching valves in a hydraulic transmission control unit in their working position. The performance of rapid shifting operations and, in particular, prompt starting after turning on the combustion engine again, is therefore at least difficult without additional measures.
To solve this problem, it is known to power the hydraulic pump no longer mechanically by the combustion engine, but by means of a separate electric motor. This, however, has the disadvantage that such an electric motor takes up additional structural space and increases manufacturing costs.
It is also known to equip motor vehicles with so-termed hybrid drives, which can be constructed and operated in various ways. In this, however, an internal combustion engine is usually combined with an electric machine.
If the electric machine is built as a starter-generator, it can be used both as the starter for the combustion engine and as the drive machine for purely electric or combined electric and combustion engine operation.
In addition, such a hybrid drive offers the pleasing possibility of operating the electric machine as a generator during non-powered driving phases (for example when the vehicle is coasting), with the aid of which electrical energy is then generated and stored intermediately in a suitable storage device for later starting of the combustion engine and/or for electric drive operation. With such a hybrid drive train, start-stop operation of the vehicle is also possible.
In this context, a variable-speed automatic transmission for hybrid-drive vehicles is known from U.S. Pat. No. 6,258,001 B1, in which, starting from the crankshaft of an internal combustion engine, a rotary oscillation damper, an electric machine, a hydrodynamic torque converter, a hydraulic pump and a planetary gearset are arranged drive-technologically one after the other. With this automatic transmission, a motor vehicle can advantageously be operated by means of the combustion engine and/or by means of the electric machine.
In relation to the powering of the transmission's hydraulic pump, this makes it possible for the pump to be driven by the combustion engine and/or by the electric motor. However, it is a disadvantage that while the combustion engine is off, although the hydraulic pump can be driven by the electric motor, the crankshaft of the combustion engine and, when the converter bridging clutch is closed, also the turbine wheel of the hydrodynamic torque converter, are driven with it in an energy-wasting way.
Against this background, therefore, it is the purpose of the present invention to propose a hybrid drive train with an automatic transmission for a motor vehicle, in which a hydraulic pump can be driven even when the combustion engine is off without using an additional electric motor and with low energy input.